1. Field of the Invention
The present invention relates to sealing for a semiconductor device.
2. Description of the Prior Art
Conventionally, the final coating of a semiconductor chip (a transistor or a semiconductor device on which transistors are integrated in a plurality of locations, hereinafter referred to as a chip) is carried out at the wafer level. Therefore, an aluminum pad section (usually 100 .mu.m.times.100 .mu.m) which is formed subsequently for wire bonding is exposed to the epoxy mold section.
For this reason, in a plastic Dual Inline Package (DIP) comprising a lead frame 37 having a die pad 28 and a stem 35, a chip 26 adhering closely to the die pad 28 and having an aluminum pad 38 for electric connection as shown in FIG. 1, it has been ascertained that silicon nitride 30 has a blocking effect against the influx 33 of moisture (humidity) through a plastic bulk 36, but it is completely ineffective against the influx 34 of moisture along a wire or against the influx 31 of moisture through a crack 32 at the interface of the lead frame 37 and the mold 36. For this reason, the aluminum pad 38 is easily corroded, so that the characteristics of the semiconductor device deteriorate, leading to reduced reliability.
Especially, for example, in the case where the 410B epoxy mold material of the Morton Co. is used for the mold section, large quantities of chlorine remain generally throughout the mold material, and chlorine ions are formed by virtue of the existence of water to react with the aluminum, so that corrosion occurs. Consequently, the aluminum becomes aluminum hydroxide, and wire breaks occur. Therefore, the reliability of the semiconductor device outstandingly deteriorates.
In addition, semiconductor device is completely defenseless against the occurrence of corrosion at the pad section due to the moisture which enters the crack 32 which is easily developed at the bonding surface of the lead frame and the epoxy mold when the frame is bent at the lead section and the tie bar is cut.
Further, when an IC chip molded with these plastics is soldered to a substrate, there is the problem that when heated to about 260.degree. C., the moisture remaining in the inner part of the plastic package turns into steam, expands in the inside of the plastic package, and cracks appear in the plastic package. In the prior art technology known to eliminate this problem, the steam is allowed to escape to the outside of the plastic package through a vent hole provided in the underside of the die pad of the lead frame, whereby the development of cracks is therefore suppressed.
However, when this type of configuration is adopted, the problem arises that when the vent holes are provided, the underside of the die pad of the lead frame is directly exposed to the outside atmosphere, so that corrosion occurs on the underside of the die pad.
Specifically, moisture from the outside atmosphere enters and concentrates in an interface between the mold and the lead frame, and reliability of the semiconductor device is decreased. In other words, cracks are produced in in the mold due to thermal expansion, resulting in corrosion, thereby deteriorating the intimate contact between the mold and the lead frame.